Factors induced by interferons (IFNs) bind to the IFN-stimulated response elements (ISREs) of many genes. In human cells treated with type I (alpha, beta) IFN, factor E is induced in about 1 min and factor M after about 1 hr. Factor G is induced after about 1 hr in cells treated with type II (gamma) IFN. G and M have very similar positions in bandshift assays, sensitivities to cycloheximide, footprints on an ISRE and relative affinities for different ISREs. Four different patterns of expression were observed in different cell lines: E,M and G strongly induced; M and G strongly but E weakly; only E and M induced; only E induced. Transcription in response to IFN alpha is initiated by E and probably maintained by M since, in fibroblasts, M is present maximally when transcription is most active and declines together with transcription. In Bristol-8 cells, where induction of M is not detected, transcription is still induced by IFN alpha and still declines in its continued presence, suggesting that M is not essential for either process. A variant ISRE with two G-to-C mutations binds E especially weakly but M and G strongly. The mutations don't change the response of a reporter gene to IFN gamma but do abolish its response to IFN alpha, suggesting that the binding of M is not sufficient for the latter. G probably acts positively, since its appearance correlates well with induction of transcription by IFN gamma. A 39-bp ISRE from the 9-27 gene binds E much better than M or G. Conversely, a 39-bp ISRE from the 6-16 gene binds M and G much better than E. Different patterns of expression of E, M, and G and different affinities of these factors for alternative ISREs may play a part in modulating the relative responses of genes to type I and type II IFNs in vivo.